1. Field of the Invention
The invention relates generally to optical transmission systems and, more particularly, to fiber laser pumps for Raman amplifiers.
2. Background of the Invention
Presently, optical transmission systems are being utilized to transport voice and/or data packets between various locations. As optical signal propagates along an optical fiber, the optical signal power degrades at a rate of about 0.2 db per km. Consequently, optical transmission systems employ a variety of optical amplifiers to amplify optical signal power propagating along the optical fiber. Raman amplifiers, for example, are a commonly used type of optical amplifiers.
There are two main Raman amplifier configurations, distributed Raman amplifiers and discrete Raman amplifiers. Discrete Raman amplifiers typically amplify an optical signal power within a generally small section of specially designed amplification fiber, such as high delta or dispersion compensating fiber packaged together with an optical pump device. A relatively small section (typically smaller than 1 km) of transmission fiber may also be utilized.
Distributed Raman amplifiers amplify optical signal power within a transmission fiber itself. More specifically, the distributed Raman amplifier utilizes a long length (about 100 km) of transmission fiber directly coupled to a pump source. The pump source generates a pump wavelength, which is used to amplify optical signal power propagating along the optical transmission fiber in a signal wavelength band. The amplified signal wavelength band is longer than pump wavelength, by approximately 13 THz.
A need exists for a Raman pump laser that can be utilized to efficiently amplify optical signal in the range of about 1525 nm to about 1625 nm, i.e., the low wavelength loss window currently utilized by the C and L bands of erbium doped fiber amplifiers (EDFAs).
The present invention relates to the utilization of thulium doped fiber lasers as pump lasers in Raman amplifiers for amplification of signals in the wavelength range traditionally covered by the C and L bands of EDFAs.
According to one embodiment of the present invention, an optical transmission system comprises: a Raman amplifier including (i) a rare earth doped optical fiber laser; (ii) a light emitting pump device optically coupled to the thulium doped fiber laser, and (iii) a Raman amplifier fiber optically coupled to the rare earth doped optical fiber laser. The light emitting pump device, in combination with the rare earth doped fiber, generates a first amplification pump wavelength received by the Raman amplifier. The first amplification pump wavelength amplifies an optical signal in the Raman amplifier.
According to one embodiment of the present invention, the rare earth fiber is a thulium doped, single mode, double clad fiber including a thulium doped core glass, an inner surrounding layer of clad glass having a lower refractive index than the core glass, and an outer surrounding layer of clad glass having a lower refractive index than the inner surrounding layer of clad glass. Preferably, the double clad fiber is pumped with light in the range of about 780 nm to 830 nm. More preferably, the double clad fiber is pumped with a broad area diode that pumps the inner cladding of the double clad fiber.
According to one embodiment of the present invention, the thulium doped optical fiber is a single clad fiber that includes a thulium doped core glass and a surrounding layer of clad glass having a lower refractive index than the core glass. Preferably, the single clad fiber is pumped at a wavelength of from about 1000 nm to about 1060 nm by a light-emitting device.
According to one embodiment of the present invention, a method of amplifying an optical signal in a Raman amplification system includes the steps of providing a thulium doped optical fiber, generating a first Raman pump wavelength in the thulium doped optical fiber, and pumping a Raman amplifier fiber with the first Raman pump wavelength. The first amplification pump wavelength provides Raman amplification of an optical signal having a wavelength in the range of from about 1530 nm to about 1625 nm. Preferably, a second Raman pump wavelength is provided by the thulium doped optical fiber to broaden the optical signal bandwidth over which reasonably flat gain can be achieved.
According to one embodiment of the present invention, an optical amplifier includes a thulium doped optical fiber and one or more low power seed sources optically coupled to the thulium doped optical fiber, for generating one or more Raman pump wavelengths in the range 1430 nm to 1530 nm. It is preferable that these seed sources provide an optical output power of less than 100 mW, and more preferable that this optical power be between 100 mW and 50 mW.
According to one embodiment of the present invention, the operating wavelengths of the thulium fiber laser are selected by fiber Bragg gratings optically coupled to the thulium doped optical fiber. The operating wavelengths are optically coupled to the Raman amplifier fiber.
According to one embodiment of the present invention, an optical transmission system comprises a Raman amplifier that includes a thulium doped optical fiber, a light emitting pump device optically coupled to the thulium doped optical fiber, and a Raman amplifier fiber optically coupled to the thulium doped optical fiber and receiving at least two different pump wavelengths. The light emitting pump device in combination with the thulium doped optical fiber generates at least two different pump wavelengths. These two different pump wavelengths amplify an optical signal in the Raman amplifier fiber. The Raman amplifier operates at a signal wavelength in the range of about 1530 nm to about 1625 nm. Preferably, the magnitude of each of the at least two different pump wavelengths is adjusted to provide different magnitudes of amplification over the signal wavelength range of the Raman amplifier and to allow for power flow due to Raman gain from the shorter pump wavelength to the longer pump wavelength.